Consolidated Statements of Changes in Stockholders' Equity (Unaudited) - USD ($) $ in Millions	Total	Preferred Stock	Common Stock	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Treasury Stock
Beginning balance at Dec. 31, 2015		$ 388.5		$ 1,072.3	$ 8,242.8	$ (372.7)	$ (1,033.6)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of Preferred Stock, Series D		0.0
Treasury Stock Transactions — Stock Options and Awards				(71.2)			76.6
Stock Options and Awards — Amortization				26.4
Stock Options and Awards — Tax Benefit				(9.0)
Net Income	$ 245.4				245.4
Dividends Declared — Common Stock					(83.9)
Dividends Declared — Preferred Stock					(5.9)
Net Unrealized Gains on Securities Available for Sale	74.7					74.7
Net Unrealized Gains (Losses) on Cash Flow Hedges	6.1					6.1
Foreign Currency Translation Adjustments	3.9					3.9
Pension and Other Postretirement Benefit Adjustments	4.1					4.1
Stock Purchased							(140.3)
Ending balance at Mar. 31, 2016	8,832.8	388.5	$ 408.6	1,018.5	8,398.4	(283.9)	(1,097.3)
Beginning balance at Dec. 31, 2016	9,770.4	882.0	408.6	1,035.8	8,908.4	(370.0)	(1,094.4)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of Preferred Stock, Series D		0.0
Treasury Stock Transactions — Stock Options and Awards				(81.8)			119.0
Stock Options and Awards — Amortization				56.2
Stock Options and Awards — Tax Benefit				0.0
Net Income	276.1				276.1
Dividends Declared — Common Stock					(89.4)
Dividends Declared — Preferred Stock					(20.7)
Net Unrealized Gains on Securities Available for Sale	19.3					19.3
Net Unrealized Gains (Losses) on Cash Flow Hedges	(5.3)					(5.3)
Foreign Currency Translation Adjustments	2.0					2.0
Pension and Other Postretirement Benefit Adjustments	2.0					2.0
Stock Purchased	(70.1)						(70.1)
Ending balance at Mar. 31, 2017	$ 9,977.7	$ 882.0	$ 408.6	$ 1,010.2	$ 9,074.4	$ (352.0)	$ (1,045.5)